1. Field of the Invention
The present invention relates to a brake force control apparatus and, more particularly, to a brake force control apparatus which is suitable for controlling a brake force of a vehicle.
2. Description of the Related Art
Conventionally, as disclosed in Japanese Laid-Open Patent Application No.4-121260, a brake force control apparatus generates a brake force greater than that of a normal condition when a brake pedal is depressed at a speed exceeding a predetermined speed. A driver of a vehicle operates a brake pedal at a high speed when the driver desires to rapidly increase a brake force. Thus, according to the above-mentioned process, the brake force intended by the driver can be generated under such circumstances. Hereinafter, the control to generate a brake fluid pressure greater than that of a normal operation as mentioned above is referred to as a brake assist control (BA control).
Additionally, the above-mentioned conventional apparatus performs an antilock brake control (ABS control) for preventing a wheel from being locked by appropriately reducing a wheel cylinder pressure of the wheel when a slip rate of the wheel becomes an excessive value during a brake operation. Thus, according to the above-mentioned conventional apparatus, when a brake operation is performed by a driver, a brake force greater than that of a normal operation can be generated without generation of an excessive slip rate in a wheel.
The above-mentioned BA control and ABS control can be achieved by an apparatus, for example, including: 1) a master cylinder; 2) a master-cut valve which controls a communication state between the master cylinder and a wheel cylinder; 3) a pump; 4) an intake valve which controls a communication state between the master cylinder and an inlet port of the pump; 5) a holding valve which controls a communication state of the wheel cylinder and each of the master cylinder and the pump; 6) a supplementary reservoir which communicates with the inlet port of the pump; and 7) a depressurizing valve which controls a communication state between the supplementary reservoir and the wheel cylinder.
According to the above-mentioned apparatus, a brake fluid in the master cylinder can be delivered by the pump by opening 4) the inlet port and actuating 3) the pump. At this time, the wheel cylinder pressure can be increased by a discharge pressure of the pump by closing 2) the master-cut valve, opening 5) the holding valve and closing 7) the depressurizing valve. Accordingly, when the above-mentioned state (hereinafter, referred to as a BA state) is established, the BA control can be performed.
Additionally, the wheel cylinder is connected to the 6) supplementary reservoir by closing 4) the intake valve and opening 7) the depressurizing valve after the above-mentioned BA state is established. Hereinafter, this state is referred to as an ABS depressurized state. When the ABS depressurized state is established, the brake fluid in the wheel cylinder is released to the supplementary reservoir and the wheel cylinder pressure is decreased. Thus, if the ABS depressurized state is established when an excessive slip rate is generated in a wheel, the ABS control can be performed while the BA control is performed.
In the above-mentioned apparatus, the inlet port of the pump is connected to the master cylinder via the intake valve and is also connected to the supplementary reservoir. Accordingly, the pump intakes brake fluid from the master cylinder or the supplementary reservoir. In the above-mentioned apparatus, the BA control is performed under a condition in which a brake pedal is deeply depressed by a driver, that is, under a condition in which a high-pressure brake fluid is supplied from the master cylinder to the inlet port of the pump. In such a condition, if the brake fluid is stored in the supplementary reservoir, the pump intakes brake fluid from the master cylinder.
Accordingly, in the above-mentioned apparatus, if the BA control and the ABS control are concurrently performed, an amount of brake fluid in the supplementary reservoir is increased each time the above-mentioned ABS depressurized state is established, and finally the supplementary reservoir may be filled up with brake fluid. When the supplementary reservoir is completely filled up with brake fluid, the brake fluid in the wheel cylinder cannot be released to the supplementary reservoir even if the depressurizing valve is opened. In this case, there is a problem in that the wheel cylinder pressure cannot be decreased even if the ABS control is required. Thus, according to the above-mentioned apparatus, when the BA control and the ABS control are concurrently performed, it is difficult to properly achieve the function of the ABS control.